Photolithography or optical lithography is a process used in semiconductor device fabrication to transfer a pattern from a photomask or reticle to the surface of a substrate. Often, crystalline silicon in the form of a wafer is used as a choice of substrate, although glass, sapphire and metal may alternatively or additionally be used, among other materials.
A typical lithography procedure would begin by depositing a layer of conductive metal or other material on the substrate. A layer of photoresist is then formed over the metal layer. A photomask is then placed between a source of illumination and the wafer to selectively expose parts of the substrate to light. The photoresist is then developed, by which areas of unhardened photoresist undergo a chemical change. After a hard-bake, subsequent chemical treatments etch away the portions of the metal layer under the developed photoresist, and then etch away the hardened photoresist, leaving the patterned metal layer.
A commonly used approach for photolithography is projection lithography, in which a desired pattern is projected from the photomask onto the wafer in a machine called a scanner. In a scanner, light from a mercury arc lamp or excimer laser is focused onto a “mask” or reticle containing the desired image. The light passes through the mask and is then focused to produce the desired image on the wafer through a reduction lens system. The reduction of the system can vary depending on design, but is typically on the order of 4×-5× in magnitude.
When the image is projected onto the wafer, the photoresist material undergoes wavelength-specific radiation-sensitive chemical reactions, which cause the regions exposed to light to be either more or less acidic. If the exposed regions become more acidic, the material is called a positive photoresist, while if it becomes less susceptible it is a negative photoresist. The resist is then developed by exposing it to an alkaline solution that removes either the exposed (positive) photoresist or the unexposed (negative) photoresist.
Lithography is used because it affords precise control over the shape and size of the objects it creates, and because it can create patterns over an entire surface simultaneously. However, the process does have its pitfalls.
For example, an important process for enhancing device performance in IC fabrication for technologies at 65 nm and beyond is surface annealing. However, surface annealing processes can cause the wafer to bend or bow as a result of stress disparities between adjacent layers formed on the substrate and in the substrate itself. The resulting wafer curvature can shift alignment marks, making subsequent lithography alignment difficult and inducing poor overlay performance.
Current scanner alignment correction includes translation, rotation, non-orthogonality and expansion. However, these corrections may not always be sufficient to correct for the wafer curvature resulting from thermal processing.